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Materials Applications (material + application)
Selected AbstractsCover Picture: Anisotropy and Dynamic Ranges in Effective Properties of Sheared Nematic Polymer Nanocomposites (Adv. Funct.ADVANCED FUNCTIONAL MATERIALS, Issue 12 2005Mater. Abstract Forest and co-workers report on p.,2029 that nematic polymer nanocomposite (NPNC) films can be processed in steady shear flows, which generate complex orientational distributions of the nanorod inclusions. Distribution functions for a benchmark NPNC (11,vol.-% of 1,nm,×,200,nm rods) are computed for a range of shear rates, yielding a bifurcation diagram with steady states at very low (logrolling) and high (flow-aligning) shear rates, and limit cycles (tumbling, wagging, kayaking) at intermediate shear rates. The orientational distributions dictate the effective conductivity tensor of the NPNC film, which is computed for all distribution functions, and extract the maximum principal conductivity enhancement (Emax, averaged in time for periodic distributions) relative to the matrix. The result is a "property bifurcation diagram" for NPNC films, which predicts an optimal shear rate that maximizes Emax. Nematic, or liquid-crystalline, polymer nanocomposites (NPNCs) are composed of large aspect ratio, rod-like or platelet, rigid macromolecules in a matrix or solvent, which itself may be aqueous or polymeric. NPNCs are engineered for high-performance material applications, ranging across mechanical, electrical, piezoelectric, thermal, and barrier properties. The rods or platelets possess enormous property contrasts relative to the solvent, yet the composite properties are strongly affected by the orientational distribution of the nanophase. Nematic polymer film processing flows are shear-dominated, for which orientational distributions are well known to be highly sensitive to shear rate and volume fraction of the nematogens, with unsteady response being the most expected outcome at typical low shear rates and volume fractions. The focus of this article is a determination of the ranges of anisotropy and dynamic fluctuations in effective properties arising from orientational probability distribution functions generated by steady shear of NPNC monodomains. We combine numerical databases for sheared monodomain distributions[1,2] of thin rod or platelet dispersions together with homogenization theory for low-volume-fraction spheroidal inclusions[3] to calculate effective conductivity tensors of steady and oscillatory sheared mesophases. We then extract maximum scalar conductivity enhancement and anisotropy for each type of sheared monodomain (flow-aligned, tumbling, kayaking, and chaotic). [source] Microporous Conjugated Poly(thienylene arylene) NetworksADVANCED MATERIALS, Issue 6 2009Johannes Schmidt Conjugated microporous polymer networks containing thienyl units are synthesized from two different monomers using oxidative polymerization. The resulting high surface area materials present interface rich, conjugated-polymer architectures, and are promising candidates for advanced material applications, such as in the field of organic electronics, for sensors and solar cells, or as polymer supports for metal catalysts. [source] Curing Dental Resins and Composites by PhotopolymerizationJOURNAL OF ESTHETIC AND RESTORATIVE DENTISTRY, Issue 6 2000JEFFREY W. STANSBURY PHD ABSTRACT The development and continued evolution of photopolymerizable dental materials, particularly dental composite restoratives, represent a significant, practical advance for dentistry. The highly successful integration of the light-activated curing process for dental applications is described in this review. The basic mechanisms by which the photoinitiators efficiently convert monomers into polymers are discussed along with the variety of factors that influence the photopolymerization process. The conventional camphorquinone-amine visible light photoinitiator system used in most dental restorative materials is illustrated in addition to some alternative initiator systems that have been studied for dental materials applications. CLINICAL SIGNIFICANCE Photopolymerization has become an integral component of the practice of dentistry. A better appreciation of the photopolymerization process as well as its potential and limitations may aid the dentist in the delivery of both esthetic and restorative dental care. [source] Origins and Applications of London Dispersion Forces and Hamaker Constants in CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2000Roger H. French The London dispersion forces, along with the Debye and Keesom forces, constitute the long-range van der Waals forces. London's and Hamaker's work on the point-to-point dispersion interaction and Lifshitz's development of the continuum theory of dispersion are the foundations of our understanding of dispersion forces. Dispersion forces are present for all materials and are intrinsically related to the optical properties and the underlying interband electronic structures of materials. The force law scaling constant of the dispersion force, known as the Hamaker constant, can be determined from spectral or parametric optical properties of materials, combined with knowledge of the configuration of the materials. With recent access to new experimental and ab initio tools for determination of optical properties of materials, dispersion force research has new opportunities for detailed studies. Opportunities include development of improved index approximations and parametric representations of the optical properties for estimation of Hamaker constants. Expanded databases of London dispersion spectra of materials will permit accurate estimation of both nonretarded and retarded dispersion forces in complex configurations. Development of solutions for generalized multilayer configurations of materials are needed for the treatment of more-complex problems, such as graded interfaces. Dispersion forces can play a critical role in materials applications. Typically, they are a component with other forces in a force balance, and it is this balance that dictates the resulting behavior. The ubiquitous nature of the London dispersion forces makes them a factor in a wide spectrum of problems; they have been in evidence since the pioneering work of Young and Laplace on wetting, contact angles, and surface energies. Additional applications include the interparticle forces that can be measured by direct techniques, such as atomic force microscopy. London dispersion forces are important in both adhesion and in sintering, where the detailed shape at the crack tip and at the sintering neck can be controlled by the dispersion forces. Dispersion forces have an important role in the properties of numerous ceramics that contain intergranular films, and here the opportunity exists for the development of an integrated understanding of intergranular films that encompasses dispersion forces, segregation, multilayer adsorption, and structure. The intrinsic length scale at which there is a transition from the continuum perspective (dispersion forces) to the atomistic perspective (encompassing interatomic bonds) is critical in many materials problems, and the relationship of dispersion forces and intergranular films may represent an important opportunity to probe this topic. The London dispersion force is retarded at large separations, where the transit time of the electromagnetic interaction must be considered explicitly. Novel phenomena, such as equilibrium surficial films and bimodal wetting/dewetting, can result in materials systems when the characteristic wavelengths of the interatomic bonds and the physical interlayer thicknesses lead to a change in the sign of the dispersion force. Use of these novel phenomena in future materials applications provides interesting opportunities in materials design. [source] Optical properties of correlated materials , Or why intelligent windows may look dirty,PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2009Jan M. Tomczak Abstract Materials with strong electronic Coulomb correlations play an increasing role in modern materials applications. "Thermochromic" systems, which exhibit thermally induced changes in their optical response, provide a particularly interesting case. The optical switching associated with the metal,insulator transition of vanadium dioxide (VO2), for example, has been proposed for use in numerous applications, ranging from anti-laser shields to "intelligent" windows, which selectively filter radiative heat in hot weather conditions. Are present-day electronic structure techniques able to describe, or , eventually even predict , such a kind of behavior? How far are we from materials design using correlated oxides? These are the central questions we try to address in this article. We review recent attempts of calculating optical properties of correlated materials within dynamical mean field theory, and summarize results for VO2 obtained within a novel scheme aiming at particularly simple and efficient calculations of optical transition matrix elements within localized basis sets. Finally, by optimizing the geometry of "intelligent windows," we argue that this kind of technique can, in principle, be used to provide guidance for experiments, thus giving a rather optimistic answer to the above questions. [source] The chromatography-free release, isolation and purification of recombinant peptide for fibril self-assemblyBIOTECHNOLOGY & BIOENGINEERING, Issue 5 2009B.M. Hartmann Abstract One of the major expenses associated with recombinant peptide production is the use of chromatography in the isolation and purification stages of a bioprocess. Here we report a chromatography-free isolation and purification process for recombinant peptide expressed in Escherichia coli (E. coli). Initial peptide release is by homogenization and then by enzymatic cleavage of the peptide-containing fusion protein, directly in the E. coli homogenate. Release is followed by selective solvent precipitation (SSP) to isolate and purify the peptide away from larger cell contaminants. Specifically, we expressed in E. coli the self-assembling ,-sheet forming peptide P11 -2 in fusion to thioredoxin. Homogenate was heat treated (55°C, 15,min) and then incubated with tobacco etch virus protease (TEVp) to release P11 -2 having a native N-terminus. SSP with ethanol at room temperature then removed contaminating proteins in an integrated isolation-purification step; it proved necessary to add 250,mM NaCl to homogenate to prevent P11 -2 from partitioning to the precipitate. This process structure gave recombinant P11 -2 peptide at 97% polypeptide purity and 40% overall yield, without a single chromatography step. Following buffer-exchange of the 97% pure product by bind-elute chromatography into defined chemical conditions, the resulting peptide was shown to be functionally active and able to form self-assembled fibrils. To the best of our knowledge, this manuscript reports the first published process for chromatography-free recombinant peptide release, isolation and purification. The process proved able to deliver functional recombinant peptide at high purity and potentially low cost, opening cost-sensitive materials applications for peptide-based materials. Biotechnol. Bioeng. 2009; 104: 973,985. © 2009 Wiley Periodicals, Inc. [source] | ||||||||||